In recent years, there has been a demand for excellent wear resistance and low rolling resistance in pneumatic tires. A known technique for satisfying these demands is to blend a styrene-butadiene copolymer or a reinforcing filler such as silica into a rubber composition constituting the cap tread of a tire. Further, to enhance the wear resistance, rubber hardness, or modulus of repulsion elasticity of the rubber composition, it has been proposed to blend a polybutadiene or silica having high reactivity, for example, but this is problematic in that the rubber strength decreases or the workability deteriorates.
Japan Unexamined Patent Publication No. 03-239737 discloses that a pneumatic tire in which a rubber composition prepared by blending a styrene-butadiene copolymer having a specific sequence of styrene units and silica is used for a tread simultaneously satisfies wet skid resistance, rolling resistance, and wear resistance. However, this rubber composition is not necessarily capable of sufficiently satisfying low heat build-up and wear resistance of a level demanded by consumers.
Japan Unexamined Patent Publication No. 57-179212 discloses that an ozone-decomposed product of a styrene-butadiene copolymer is analyzed by gel permeation chromatography (GPC), and the content ratio of a long-chain styrene block and the content ratio of a simple chain with one styrene unit with respect to the total styrene content are measured. A styrene-butadiene copolymer in which the long-chain styrene block content is not greater than 5 wt. % and the simple chain content with one styrene unit is not less than 50 wt. % with respect to the total styrene content in the styrene-butadiene copolymer, and the total styrene content is from 10 to 30 wt. % of the styrene-butadiene copolymer is then further disclosed. However, a rubber composition made of this styrene-butadiene copolymer is not necessarily capable of sufficiently enhancing the wear resistance and low hysteresis loss thereof. In addition, in the GPC-based analysis in Japan Unexamined Patent Publication No. 57-179212, even in a case where the ozone-decomposed product of the styrene-butadiene copolymer can be classified based on the number of styrene units and a chain made of only styrene units, the number of units derived from 1,2-bonds of butadiene in the ozone-decomposed product cannot be identified. Further, in the GPC-based analysis of Japan Unexamined Patent Publication No. 57-179212, the ratio of the styrene content in the decomposition product to the total styrene content is calculated, and the amount of each decomposition product is not accurately quantified.